Infra-red communications links are used in many applications. Probably the most common is the remote control of television receivers. In such systems, pulse coded infra-red signals are generated in a hand-held, battery powered control unit. Different codes are generated in accordance with push-button inputs to control, for example, channel switching and volume level. Other applications of such hand-held devices include the control of various other domestic appliances and the remote control of mobile models.
In another group of infra-red transmission applications, a continuous pulse modulated beam is generated, and a sensor device determines when the beam is interrupted. These applications include intruder detection and article counting in, for example, manufacturing lines.
It should be noted that in the first of the above mentioned groups of applications, that is the television and domestic appliance controls, the number of transmissions per day is somewhat limited, and can be counted in tens. In the second group, the transmitter systems are normally energised from an a.c. line source. A problem occurs when a battery powered control unit is to be used for thousands of operations per day. This problem is, of course, battery life. Such a problem occurs when the transmitter system forms part of a keyboard entry device for a microcomputer, whereby the coupling between the keyboard device and the microcomputer is via an infra-red link. In such an arrangement, most keystrokes will generate a data set for transmission and at an average keystroke speed of, for example, fifty strokes per minute, three thousand characters are generated each hour. Comparing this with the television controller system usage, it is seen that the daily usage varies by a factor of greater than 1000:1. Thus, in the keyboard entry system, battery life is an important consideration, and any reduction in current consumption is highly significant.
U.S. Pat. No. 4,313,227 (Eder) shows a system in which data entry devices are coupled to a microcomputer through infra-red links. In that system, the main keyboard is in the microcomputer, and portable control units, each comprising a joystick device and a keypad, are coupled via infra-red links. No specific battery consumption minimising arrangement is shown for the control units. This is most probably because they are games input devices and, therefore, are not designed for the prolonged use expected from a full keyboard device.
It is an object of the present invention to provide an infra-red data transmission system for coupling data from a battery powered keyboard entry device to a microcomputer with reduced battery power consumption.